Wikipedia went offline at midnight Eastern time, and will not become available again until midnight on January 19, meaning online users will not be able to read any of the encyclopaedia entries over the 24 hours.

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Users visiting the site are instead directed to a message saying: "For over a decade, we have spent millions of hours building the largest encyclopaedia in human history.

"Right now, the US Congress is considering legislation that could fatally damage the free and open internet. For 24 hours, to raise awareness, we are blacking out Wikipedia."

In information posted on the site, Wikipedia said that it is concerned SOPA and PIPA will "severely inhibit people's access to online information", noting that this is not just a problem affecting the US, but also "everyone around the world".

The site, founded by Jimmy Wales, said that the two pieces of legislation are "badly drafted" and won't stop copyright infringement, but will cause serious damage to the free and open internet.

Wikipedia is among a group of US technology firms campaigning against SOPA, which aimed to 'throttle' sites suspected of copyright infringement by denying them prominence on search engines or access to online payment services.

The tech giants claim that the legislation is a new and potentially dangerous form of online censorship, as it would effectively involve someone deciding which sites were given prominence on the internet, threatening the core principle of 'net neutrality'.

The US government has been under severe pressure from Hollywood and the music industry to crack down on online piracy and increase regulation of the internet.

But at the weekend, the Obama administration sided with the technology industry, saying that SOPA will not be passed in its current form.

However, the broadly similar PIPA bill was approved by the Senate committee last year, and is due to go before the full Senate on January 24.

"There are signs PIPA may be debated on the Senate floor next week. Moreover, SOPA and PIPA are just indicators of a much broader problem. We are already seeing big media calling us names," said Wikipedia.

"In many jurisdictions around the world, we're seeing the development of legislation that prioritises overly-broad copyright enforcement laws, laws promoted by power players, over the preservation of individual civil liberties. We want the internet to be free and open, everywhere, for everyone."

Other companies have joined Wikipedia in its protest by also voluntarily shutting down their sites, including Reddit and Boing Boing.

In a statement on its website, Boing Boing said: "Boing Boing is offline today, because the US Senate is considering legislation that would certainly kill us forever.

"The legislation is called the Protect IP Act (PIPA), and would put us in legal jeopardy if we linked to a site anywhere online that had any links to copyright infringement.

"This would unmake the web, just as proposed in the Stop Online Piracy Act (SOPA). We don't want that world. If you don't want it either, visit AmericanCensorship.org for instructions on contacting your Senator. The Electronic Frontier Foundation has more information on this and other issues central to your freedom online."

Google has not opted to join Wikipedia in the shutdown protest, despite being very vocal in its opposition to SOPA. But the site has added a protest link to its search engine.

"Like many businesses, entrepreneurs and web users, we oppose these bills because there are smart, targeted ways to shut down foreign rogue websites without asking American companies to censor the internet," a Google spokesperson said.

"We will be joining many other tech companies to highlight this issue on our US home page."

Despite its online shut down, Wikipedia is still accessible in some forms, including on mobile devices and smartphones. You can also view the site normally by completely disabling JavaScript in your browser.